The present invention concerns a FePt magnet and its manufacturing method. More concretely, the invention concerns a strong and small FePt magnet that has extremely good values of both of coercive force and maximum energy product, and its manufacturing method.
The Conventional Technique
In recent years, permanent magnets have been utilized not only in conventional motors, but also in medical devices that are used in a living body, such as dental magnetic attachments. For use in a living body, safety of the material is important. It is also required to demonstrate a strong magnetic force in the volume as small as possible in order to avoid burdening the living body.
In addition, research and development are being carried out for realization of so-called micro-machines. Micro-machines are expected to lead to a realization of medical treatment with fewer burdens on a living body. The development of a miniature, strong permanent magnet which has the size of not more than millimeter order and has high corrosion resistance, is required for the realization of a micro-machine.
Rare-earth magnets, of which NdFeB is representative, have been developed for and are currently widely used as high performance permanent magnets for motor and other common applications of magnets.
However, a rare-earth magnet can easily be oxidized as it has poor corrosion resistance, and as a result it cannot always be applied to the above-mentioned kinds of applications. For example, in medical devices that are used in a living body such as dental magnetic attachments, the direct use of a rare-earth magnet is difficult because of corrosion.
Consequently, in such cases, the use of a rare-earth magnet must be accompanied by complicated measures, such as a corrosion-resistant coating or containment of the magnet in a corrosion-resistant case, and it is not very easy to guarantee their corrosion resistance. In addition, such a coating sometimes brings resistance in the magnetic circuit, thereby preventing the original characteristics of the magnet from being exhibited. An example of measures against corrosion of a rare-earth magnet has been disclosed in Japanese laid-open patent publication number 11-137576.
Another demerit of a rare-earth magnet is that it is so fragile that it can easily be broken during processing, handling or use. For this reason, it is very difficult that rare-earth magnets are mechanical processed into minute, sub-millimeter sized parts, such as the above-mentioned micro-machines. Moreover, volumes of those minute parts are so small that even a small degree of oxidization on the surface can significantly effect their magnetic characteristics. Thus there are a number of problems in applying a rare-earth magnet to minute parts in terms of corrosion resistance.
On the other hand, a platinum alloy magnet such as CoPt or FePt is superior to a rare-earth magnet in terms of corrosion resistance and processing convenience. These alloys have excellent corrosion resistance, as they contain a large amount of platinum. Platinum alloy magnets also have excellent strength and toughness that lessen their chances of being broken.
FePt alloy is known to demonstrate especially good magnetic characteristics. A FePt alloy in an ordered phase demonstrates permanent magnetic characteristics, and has a CuAu (L10) type of face-centered tetragonal structure. The ordered phase can be obtained by employing the appropriate heat treatment to an alloy in an unordered phase (face-centered tetragonal structure, A-1 type). The FePt magnet mentioned above is known to have a degree of crystal magnetic anisotropy comparable to that of a rare-earth magnet (O. A. Ivanov et al, Phys. Met. Metallog. Vol. 35, p81, 1973) and is expected to have potentially very excellent magnetic characteristics.
A FePt alloy can demonstrate almost the same degree of corrosion resistance as platinum if it contains as much as 70 mass % platinum (Journal of the Japanese Society of Magnetic Applications in Dentistry, Vol. 1, No. 1, p. 14, 1982). Consequently, it is a suitable material especially for minute size magnets with high corrosion resistance.
However, these platinum alloy magnets have only achieved considerably lower magnetic characteristics compared to the rare-earth magnet.
For example, for dental use, manufacturing of FePt alloy parts by melt-cast method was attempted (Journal of the Magnetics Society of Japan, Vol. 21, p. 377-380, 1997). In the results of this study, value for maximum energy product (BH)max was reported to be 127.32 kJ/m3 (16 MGOe; 1 GOe=79.5774xc3x9710xe2x88x924 J/m3, conversion used throughout), and value for coercive force iHc was reported to be 318.30 kA/m (4 kOe:10Oe=79.5774 A/m, conversion used throughout), respectively. These are quite low compared to the magnetic characteristics of a rare-earth magnet.
A coercive force as low as 318.23 kA/m will become a serious problem when the alloy is manufactured into micro-sized parts, causing degradation in its magnetic characteristics, and yielding it unable to resist a demagnetizing field.
It has recently been reported that thin film FePt alloy demonstrates a remarkably high coercive force by means of sputtering.
The first report about thin film FePt alloy was by Aboaf (IEEE, Trans, MAG-20, p. 1642, 1984). According to this report, dependence of iHc on the composition was found, and the maximum iHc value for an equi-atomic FePt alloy was reported to be 843.52 kA/m (10.6 kOe). This report is noteworthy because it suggests that FePt might intrinsically possess good magnetic characteristics. Additionally, in terms of cost and simplicity of manufacturing miniature magnetic parts, for use in a micro-machine for example, a sputtering process, which is a film-growth process, is more desirable than a bulk process in which bulk material is mechanically processed to a predetermined size.
Aboaf""s above-mentioned report concerns quite a thin film of 300-400 nm (3000-4000 xc3x85), and it is necessary to make a thicker film in order for the alloy to be practical as a permanent magnetic part.
A Problem to Solve in the Invention
However, when the thickness of a film was increased in a sputtering process, a deterioration of the magnetic characteristics, especially in its coercive force, was found by one of the inventors (Journal of the Magnetics Society of Japan, Vol. 24, No. 4-2, p. 927, 2000). According to the report, the coercive force was measured as 716.20 kA/m (9 kOe) at a thickness around 0.5 xcexcm, and decreased as the thickness of the film was increased, to not more than 397.89 kA/m (5 kOe) at a thickness of 100 xcexcm. The decrease in coercive force was accompanied with a decrease of a maximum energy product from 127.32 kJ/m3 (16 MGOe) to as low as 79.58 kJ/m3 (10 MGOe). Thus it became apparent that a sputtering process, which had been thought to be efficient for an improvement in coercive force, was inefficient when the thickness of the film was increased to a practical range.
Because of the above evaluation, sufficient magnetic characteristics could not be achieved when miniature magnetic parts were manufactured from FePt alloy.
Sufficient magnetic characteristics are considered to be maximum energy product (BH)max values of not less than 159.15 kJ/m3 (20 MGOe) and coercive force (iHc) values of not less than 557.04 kA/m (7 k Oe), for a relatively small film thickness of 1 xcexcm. For film thickness of 30 xcexcm, it is more desirable that values for maximum energy product (BH)max are not less than 119.37 kJ/m3 (15 MGOe) and values for coercive force (iHc) are not less than 39 7.89 kA/m (5 koe) respectively, taking into account practical application to permanent magnetic parts.
Based on the circumstances stated above, the current invention is intended to provide an FePt alloy material that has good values for both maximum energy product and coercive force, and whose coercive force does not decrease with increased film thickness when manufactured by a film-growing process such as sputtering, thus allowing it to maintain a high maximum energy product.
Means to Resolve the Problem
Making a detailed study on a FePt alloy, the inventors found out that a small additive amount of a suitable third element to a FePt alloy would result in not only an improvement in its magnetic characteristics, but also an expression of a stable coercive force even with increased film-thickness leading to the ability to express a large maximum energy product even in a thick film state.
Although the reason is not completely clear why addition of a suitable third element to a FePt alloy brings about an improvement in its magnetic characteristics, through the discovery of a close relationship between coercive force and crystal particle size, the inventors consider that addition of the third element brings about a reduction in the crystal particle size, leading to an improvement in the magnetic characteristics. The following is an explanation how the invention has come to be made.
For bulk state FePt binary alloy manufactured by melting and casting and then heat treatment, the influence of the composition and heat treatment has been investigated and it has been found that the alloy displays maximum values in both coercive force and maximum energy product when it is composed of 3 8.5 atomic % Pt-Fe. However, as mentioned above, the coercive force is at most 318.31 kA/m (4 kOe), which is quite low. The crystal particle size is in the hundreds of xcexcm.
On the other hand, the crystal particle size of a sputtered FePt alloy film that has a high coercive force has been reported to be about 0.05-0.2 xcexcm. It can therefore be presumed that crystal particle size has a great influence on coercive force.
From results of an investigation of a relationship between film-thickness of a FePt alloy made by sputtering and crystal particle size, the inventors found that crystal particle size increases with increased film thickness and concluded that a decrease in coercive force is caused by the increased crystal particle size.
Addition of a small amount of a third element other than Fe and Pt was attempted as a means to inhibit an increase in crystal particle size, and from the results of repeated tests it turned out that an addition of one or more elements selected from the group consisting of IVa, Va, IIIb, and IVb elements (IUPAC) is effective.
Among the elements stated above, an addition of one or more elements selected from the group consisting of C, B, Si, Al, Ti and Zr is even more effective.
The addition of a single or compound addition of these elements inhibits crystal particle growth, bringing about an excellent coercive force. A stable coercive force enables a high maximum energy product to be expressed.
The inventors also found out that for film thickness ranging up to 100 xcexcm, the average crystal particle size that satisfies values for coercive force (iHc) of not less than 397.89 kA/m (5 kOe) and values for maximum energy product (BH)max of not less than 119.37 kJ/m3 (15 MGOe), respectively, was not more than 0.3 xcexcm. The smaller crystal particle size is, the higher the coercive force and maximum energy product that can be achieved. Crystal particle size should ideally be not more than 0.1 xcexcm, and more desirably, not more than 0.05 xcexcm.